If the PK fields are not selected as visible when you go in the diagram of that perspective, you won´t see the relation between the tables, all of them are shown "relationless"
Lets say you create a perspective with the following tables
They are related by clientkey
lets say you hide the clientkey, as it´s a key not usefull for the user.
When you go into the Sales perspective, you dont see any relation between tables, you need to go to the "default/no perspective" to see it.